Surface Attachment of Particles to Cellulose Ester Fibers

ABSTRACT

This invention pertains to the surface attachment of particles, such as metal oxides, to cellulose ester fibers. The particles are applied to the surface of the cellulose ester fibers using a protic liquid that is substantially free of plasticizer.

FIELD OF THE INVENTION

The invention generally relates to a method of attaching particles, such as metal oxides, to the surface of cellulose ester fibers.

BACKGROUND OF THE INVENTION

Typical cigarette filters are made from a continuous-filament tow band of cellulose acetate-based fibers, called cellulose acetate tow, or simply acetate tow. The use of acetate tow to make filters is described in various patents, and the tow may be plasticized. See, for example, U.S. Pat. No. 2,794,239.

The conversion of acetate tow into cigarette filters may be accomplished by means of a tow conditioning system and a plugmaker, as described, for example, in U.S. Pat. No. 3,017,309. The tow conditioning system withdraws the tow from the bale, spreads and de-registers (“blooms”) the fibers, and delivers the tow to the plugmaker. The plugmaker compresses the tow, wraps it with plugwrap paper, and cuts it into rods of suitable length. To further increase filter firmness, a non-volatile solvent may be added to solvent-bond the fibers together. These solvent-bonding agents are called plasticizers in the trade, and historically have included triacetin (glycerol triacetate), diethylene glycol diacetate, triethylene glycol diacetate, tripropionin, acetyl triethyl citrate, and triethyl citrate. Waxes have also been used to increase filter firmness. See, for example, U.S. Pat. No. 2,904,050.

Conventional plasticizer bonding agents work well for bonding and selective filtration. However, plasticizers typically are not water-soluble, and the fibers will remain bonded over extended periods of time. In fact, conventional cigarette filters can require years to degrade and disintegrate when discarded, due to the highly entangled nature of the filter fibers, the solvent bonding between the fibers, and the inherent slow degradability of the cellulose acetate polymer. Attempts have, therefore, been made to develop cigarette filters having improved degradability.

Research Disclosure No. 38,626 (pp. 375-77, June 1996) reported that using plasticizers to form cigarette filters from acetate tow decreases the filters' degradation by holding the fibers together. The Disclosure, however, noted that simply omitting the plasticizer would not allow the filters to degrade rapidly in the environment due to fiber entanglement. To solve this problem, the Disclosure proposed fibers that would significantly reduce entanglement when wet.

Despite recent efforts, there remains a need in the art for degradable cellulose ester fibers, such as those used in cigarette filters, and especially those that may be fabricated without using plasticizers or having reduced entanglement when wet.

The present invention aims to address this need as well as others, which will become apparent from the following description and the appended claims.

SUMMARY OF THE INVENTION

The invention is as set forth in the appended claims.

Briefly, in one aspect, the present invention provides a method of attaching metal oxide particles to the surface of cellulose ester fibers. The method comprises contacting cellulose ester fibers with a mixture comprising metal oxide particles dispersed in a protic liquid to attach the metal oxide particles on the surface of the fibers. The mixture is substantially free of plasticizer.

In another aspect, the invention provides for cellulose ester fibers made by the method according to the invention.

In yet another aspect, the invention provides for a cigarette filter which comprises the cellulose ester fibers made by the method according to the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an image from a scanning electron microscope (SEM) of the TiO₂ particles attached to the cellulose acetate fiber surface from Example 1.

FIG. 2 shows an image from a SEM of the ZnO particles attached to the cellulose acetate fiber surface from Example 2.

DETAILED DESCRIPTION OF THE INVENTION

It has been surprisingly discovered that particles, such as metal oxides, may be attached to the surface of cellulose ester fibers without the use of a plasticizer.

Thus, in one aspect, the present invention provides a method of attaching metal oxide particles to the surface of cellulose ester fibers. The method comprises contacting cellulose ester fibers with a mixture comprising metal oxide particles dispersed in a protic liquid to attach the metal oxide particles on the surface of the fibers. The mixture is substantially free of plasticizer.

As used herein, the term “plasticizer” is intended to describe a solvent that, when applied to cellulose ester fibers, solvent-bonds the fibers together. Examples of plasticizers include triacetin (glycerol triacetate), diethylene glycol diacetate, triethylene glycol diacetate, tripropionin, acetyl triethyl citrate, triethyl citrate, and mixtures thereof with one or more polyethylene glycols. By “substantially free,” it is meant that a plasticizer is not present in an amount that appreciably hinders the rate of degradation of the cellulose ester fibers compared to the rate of fiber degradation in the absence of the plasticizer. Preferably, the mixture is free of plasticizer.

The metal oxide particles that may be used in the present invention is not particularly limiting. The metal of the metal oxide may be selected from Groups 1-16 of the Periodic Table of Elements. Preferred metals for the metal oxides include titanium, zinc, and aluminum. The metal oxides may be mono-metal or mixed metal oxides, such as a bimetal oxide.

A preferred class of metal oxides particles includes those that are photoactive agents. As used herein, the term “photoactive agent” refers to an agent that, when applied to a cellulose ester fiber, increases the rate at which the fiber degrades upon exposure to UV radiation. Photoactive agents useful according to the invention include titanium dioxides, although other photoactive metals or metal compounds may likewise be used. The titanium dioxide particles may be in rutile or anatase form, or a mixture of the two crystalline forms in the same particle. The amount of anatase phase present in the mixed-phase particles may vary, for example, from 2 to 98 wt %, from 15 to 95 wt %, or from 50 to 95 wt %, as measured using x-ray diffraction techniques. The rutile phase in the mixed-phase particles may likewise vary in a similar manner, for example, from 2 to 98 wt %, from 15 to 95 wt %, or from 50 to 95 wt %, as measured using x-ray diffraction techniques. These mixed-phase particles are especially suitable at enhancing fiber degradation.

The titanium dioxide particles may be prepared by a variety of methods known in the art, including high-temperature hydrolysis. The particles may also be obtained commercially.

The metal oxide particles for use in the present invention may vary in size, for example, from 1 nm to 50 microns in diameter. Preferred metal oxide particle diameters include, for example, from 1 nm to 250 nm and from 5 to 50 nm.

The surface area of the metal oxide particles may also vary over a wide range, such as, for example, from 1 to 400 m²/g, as measured by the BET surface area method. Preferably, the particles have a surface area in the range of, for example, 10 to 300 m²/g or 10 to 150 m²/g.

The protic liquid for use in the present invention may be any compound that has a hydrogen atom bonded to an oxygen, provided that it does not dissolve the cellulose ester fibers. Examples of protic liquids include water, most alcohols, formic acid, and acetic acid. Water, lower alcohols (e.g., those containing C₁-C₄), and mixtures thereof are preferred protic liquids. Examples of the lower alcohols include methanol, ethanol, isopropanol, and n-butanol.

The mixture comprising metal oxide particles dispersed in the protic liquid may be prepared in any matter. For example, the metal oxide particles may be added to the protic liquid, and the mixture may be shaken, stirred, or sonicated to disperse the particles in the liquid.

The amount of metal oxide particles dispersed in the protic liquid may vary over a wide range. For example, the mixture may contain from 1 to 1,000 ppm of the metal oxide particles. As another example, the mixture may contain from 10 to 500 ppm of the metal oxide particles.

According to the method of the invention, the cellulose ester fibers are brought into contact with the mixture for a sufficient amount of time for the particles to deposit or attach themselves on the surface of the fibers. The contact time may vary over a wide range, for example, from 1 second to 1 hour. The contacting step may be carried out in any manner. For example, the fibers may be dipped into the mixture and allowed to soak for a desired amount of time. Alternatively, the fibers may be pulled through the mixture until a desired amount of particles are deposited on the fibers.

As used herein, the term “cellulose ester fiber” means a fiber formed from one or more cellulose esters, such as cellulose acetate, for example, by melt-spinning or solvent-spinning. The cellulose esters useful in the present invention include cellulose acetates, cellulose propionates, and cellulose butyrates with varying degrees of substitution, as well as mixed esters of these, e.g., cellulose acetate propionate, cellulose acetate butyrate, and cellulose acetate propionate butyrate. The cellulose ester may be a secondary cellulose ester. Examples of suitable cellulose esters include those described in U.S. Pat. Nos. 1,698,049; 1,683,347; 1,880,808; 1,880,560; 1,984,147; 2,129,052; and 3,617,201; the entire contents of which are hereby incorporated by reference.

Although cigarette filters are traditionally made with cellulose acetate fibers, the invention is not limited to traditional esters or to cigarette filters. Further, while the typical degree of substitution per anhydroglucose unit (DS/AGU) of acetate for cigarette filters is about 2.45, filters may be readily constructed with a range of acetyl levels, such as from 1.5 to 2.8, or from 1.8 to 2.7, or from 1.9 to 2.5, or for example, an average DS/AGU of about 2.0. Lower DS/AGU values may provide faster degradation.

As noted, the cellulose ester can be spun into fiber, for example, by melt-spinning or by spinning from an appropriate solvent (e.g., acetone, acetone/water, tetrahydrofuran, methylene chloride/methanol, chloroform, dioxane, N,N-dimethylformamide, dimethylsulfoxide, methyl acetate, ethyl acetate, or pyridine). When spinning from a solvent, the choice of solvent depends upon the type of ester substituent and upon the desired DS/AGU. A suitable solvent for spinning fiber is acetone containing from 0 to 30 wt % of water. For cellulose acetate having a DS/AGU of 2.4 to 2.6, the spinning solvent can be acetone containing less than 3 wt % water. For cellulose acetate having a DS/AGU of 2.0 to 2.4, the spinning solvent can be acetone containing 5-15 wt % of water. For cellulose acetate having a DS/AGU of 1.7 to 2.0, the spinning solvent can be acetone containing 15-30 wt % of water.

When melt-spinning fibers, the cellulose ester may have a melt temperature, for example, from 120° C. to 250° C., or from 180° C. to 220° C.

The cellulose ester fibers for use in the present invention may be continuous fibers, or may be staple fibers having a shorter length, rendering the fibers more susceptible to degradation. Thus, the staple fibers may have a length from 3 to 10 mm, or from 4 to 8 mm. The staple fibers may likewise be randomly oriented.

The cellulose ester fibers may be crimped and have, for example, from 4-20 crimps per inch, or from 10 to 15 crimps per inch. The fibers may have a denier/filament (DPF), for example, of 20-0.1, or from 5-1.5 DPF. For processing, the fibers may optionally contain lubricants or processing aids such as mineral oil, used in an amount from 0.1 to 3%, or from 0.3 to 0.8% by weight.

After contacting the fibers with the mixture of metal oxide particles and protic liquid, liquid on the surface of the fibers may be removed. This optional removal step may be performed in any manner. For example, the excess liquid may be allowed to drain off of the fibers. The excess liquid may also be wiped off or pressed off of the fibers. The excess liquid may also be removed by evaporation, optionally at elevated temperature such as in an oven at 50 to 100° C. Any of these techniques may be used in combination to remove excess liquid from the fibers.

If the fibers have excess or unattached metal oxide particles on their surface, those particles may optionally be removed. The excess or unattached particles may be removed by any manner. For example, the fibers may be contacted with a protic liquid without metal oxide particles or with a lower concentration of metal oxide particles than the original treating mixture. The excess particles may also be removed by motion, such as centrifugal or vibratory. Any of these techniques may be used in combination to remove excess, loosely attached, or unattached particles from the fibers, if desired.

Optionally, in lieu of or in addition to the preceding post-contacting steps, the fibers may be dried to produce finished fibers with metal oxide particles attached to their surface. Residual liquid in the fibers may be removed in this step. The drying step may be performed in any matter. Preferably, the fibers are dried in an oven at elevated temperatures such as from 50 to 150° C.

Any desired amount of the metal oxide particles may be attached to the surface of the fibers using the method of the invention. For example, the finished fibers may contain from 0.01 to 10 wt % of metal oxide particles, while other amounts are possible.

In another aspect, the invention relates to cellulose ester fibers prepared using the method of the invention. In one embodiment, the fibers are free of plasticizer.

The fibers produced according to the invention are particularly useful in cigarette filters. The fibers according to the invention may be formed into cigarette filters according to methods known in the art.

As used herein, the indefinite articles “a” and “an” mean one or more, unless the context clearly suggests otherwise. Similarly, the singular form of nouns includes their plural form, and vice versa, unless the context clearly suggests otherwise.

While attempts have been made to be precise, the numerical values and ranges described herein should be considered to be approximations. These values and ranges may vary from their stated numbers depending upon the desired properties sought to be obtained by the present invention as well as the variations resulting from the standard deviation found in the measuring techniques. Moreover, the ranges described herein are intended and specifically contemplated to include all sub-ranges and values within the stated ranges. For example, a range of 50 to 100 is intended to include all values within the range including sub-ranges such as 60 to 90 and 70 to 80.

This invention can be further illustrated by the following examples of preferred embodiments thereof, although it will be understood that these examples are included merely for purposes of illustration and are not intended to limit the scope of the invention. Unless otherwise indicated, all percentages are by weight.

EXAMPLES General Surface Attachment Procedure

The cellulose acetate fiber samples with surfaced attached metal oxide particles were prepared by weighing 1 gram of cellulose acetate tow. A 0.01% w/w mixture of metal oxide particles in a protic liquid was prepared. The mixture was sonicated for 30 minutes to disperse the particles in the liquid. The cellulose acetate tow with no additive was added to the mixture and allowed to soak for 30 minutes. Tow samples were removed from the dispersion, and the excess water was drained off the sample. The tow was then transferred to a 75° C. oven. The samples were left in the oven for 1 hour. After 1 hour, the samples were removed from the oven and placed in deionized H₂O. The samples were sonicated for 30 minutes so as to remove excess or unattached particles from the surface of the cellulose acetate fibers. After sonicating, the fibers were transferred to an oven at 100° C. The cellulose acetate tow was left in the oven until dry.

Example 1

Evonik/Degussa P25 TiO₂ particles were dispersed in Nanopure H₂O (18MΩ.cm) and attached to the surface of cellulose acetate fibers according to the general procedures described above.

The P 25 TiO₂ particles were an ultrafine-size, uncoated mixed-phase TiO₂ having an average particle size of about 20 nm.

FIG. 1 shows an image from a scanning electron microscope (SEM) of the P25 TiO₂ particles attached to the cellulose acetate fiber surface from Example 1.

Example 2

ZnO particles were dispersed in isopropanol and attached to the surface of cellulose acetate fibers according to the general procedures described above.

The ZnO particles were obtained from Aldrich and had an average particle size of less than 1 micron.

FIG. 2 shows an image from a SEM of the ZnO particles attached to the cellulose acetate fiber surface from Example 2.

Examples 3-6 Photodegradation Testing of Surface Attached Evonik/Degussa P25 TiO₂ Particles

To test photodegradation of the cellulose acetate fibers with the surface attached Evonik/Degussa P25 TiO₂ particles, the samples were placed in a benchtop weatherometer. The weatherometer parameters were set as follows:

-   -   SunTest CPS+calibrated to have an irradiance value of 0.35±0.05         W/cm² and a temperature value of 55±3° C. during testing.         Calibration checked weekly.     -   Sample exposure program:         -   Light Sequence—702 minutes with Xe lamp on with heat control             activated         -   Prior to flood sequence—four 16 mm stainless balls are added             to cups and placed on a orbital shaker as a agitation step         -   Flood sequence—18 minutes with lamp off

Examples 3-6 below were tested in the weatherometer.

Example 3 (Comparative)

Acetate tow was made with cellulose acetate fibers containing no pigment in the fibers. The weathering results are shown in Table 1.

Example 4 (Comparative)

Acetate tow was made with cellulose acetate fibers containing pigment size (˜200 nm) and coated TiO₂ particles (Kronos 1071) in the fibers. This example was prepared to understand the effect of size and coating of the TiO2 on photodegradation. The weathering results are shown in Table 1.

Example 5

Acetate tow was made with cellulose acetate fibers containing no pigment in the fibers with photoactive TiO₂ particles (Degussa Evonik P25) attached to the fiber surface using the general procedures described above. The TiO₂ particles used in this example were an ultrafine size and uncoated mixed-phase TiO₂. The weathering results are shown in Table 1.

Example 6

Acetate tow was made with cellulose acetate fibers containing pigment size (˜200 nm) and coated TiO₂ (Kronos 1071) in the fibers with photoactive TiO₂ particles (Degussa Evonik P25) attached to the fiber surface using the general procedures described above. The TiO₂ particles used in this example were an ultrafine size and uncoated mixed-phase TiO₂.

This example was prepared to understand the effect of size and coating of the TiO₂ particles on photodegradation, and how photoactive TiO₂ on the surface can change the rate of photodegradation. The weathering results are shown in Table 1.

TABLE 1 Example No. 3 4 (Comparative) 5 (Comparative) 6 Time Weight Lost Weight Lost Weight Lost Weight Lost (Weeks) (%) (%) (%) (%) 0 0 0 0 0 1 0.3 12.9 0.5 10.8 2 1.2 27.2 1.5 23.5 3 2.6 40.5 3.3 35.6 4 6.2 52.3 5.9 45.1 5 15.3 61.2 10.0 52.7 6 33.5 67.2 15.3 60.4

As shown in Table 1, the two different acetate tows without the surface attached (Examples 3 and 4) did not degrade as fast the comparable acetate tows with the photoactive TiO₂ attached to the fiber surface. After 6 weeks, Example 3 displayed a weight loss of 33%, while Example 5 showed a 67% weight loss. After 6 weeks, Example 4 displayed a weight loss of 15%, while Example 6 had a weight loss of 60%.

The data in Table 1 show that adding the photoactive TiO₂ to the fiber surface according to the invention can increase the degradation rate of conventional acetate tow.

Example 7 Stability Testing of Surface Attached Particles

Cellulose acetate fiber samples with surfaced attached particles were prepared according to the general procedures described above to determine the stability of the particles on the fiber surface. Four fiber samples were prepared with TiO₂ particles, while another four fiber samples were prepared with ZnO particles. The surface-attached fiber samples were placed in individual beakers of water and placed in a sonicator. The samples were then sonicated up to 5 hours. The samples were then analyzed by ICP-OES to determine Zn and Ti content on the fibers. The results of the ICP-OES analysis for the sonicated samples are shown in Table 2.

TABLE 2 Concentration of TiO₂ Concentration of ZnO Sonication Particles on Fiber Particles on Fiber Time (hrs) Sample (wt %) Sample (wt %) 0 1.36 3.37 1 1.66 5.08 3 1.50 6.78 5 1.39 7.93

The data in Table 2 show that essentially no change was found for the concentration of TiO₂ and ZnO particles attached to the surface of the cellulose acetate fibers after 5 hours of sonication.

The invention has been described in detail with particular reference to preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention. 

We claim:
 1. A method of attaching metal oxide particles to the surface of cellulose ester fibers, comprising: contacting cellulose ester fibers with a mixture comprising metal oxide particles dispersed in a protic liquid to attach the metal oxide particles on the surface of the fibers, wherein the mixture is substantially free of plasticizer.
 2. The method according to claim 1, wherein the protic liquid is water, alcohol, or mixtures thereof.
 3. The method according to claim 1, wherein the metal oxide particles comprise mono-metal oxides or bi-metal oxides.
 4. The method according to claim 1, wherein the metal oxide particles are photoactive.
 5. The method according to claim 4, wherein the metal oxide particles comprise rutile titanium dioxide or anatase titanium dioxide, or mixtures of rutile titanium dioxide and anatase titanium dioxide.
 6. The method according to claim 4, wherein the metal oxide particles comprise mixed-phase titanium dioxide particles.
 7. The method according to claim 6, wherein the mixed-phase titanium dioxide particles comprise an anatase phase in an amount ranging from 50 to 98 wt % and a rutile phase in an amount ranging from 2 to 50 wt %.
 8. The method according to claim 1, wherein the metal oxide particles have a diameter of 1 nm to 50 microns.
 9. The method according to claim 1, wherein the metal oxide particles have a diameter of 1 nm to 250 nm.
 10. The method according to claim 1, wherein the metal oxide particles have a diameter of 5 nm to 50 nm.
 11. The method according to claim 1, wherein the metal oxide particles have a surface area of 1 to 400 m²/g.
 12. The method according to claim 1, wherein the metal oxide particles have a surface area of 10 to 350 m²/g.
 13. The method according to claim 1, wherein the metal oxide particles have a surface area of 10 to 150 m²/g.
 14. The method according to claim 1, wherein after the contacting step, the fibers comprise from 0.01 to 10 wt % of the metal oxide particles.
 15. The method according to claim 1, wherein the cellulose ester fibers comprise cellulose acetate fibers, cellulose propionate fibers, cellulose butyrate fibers, cellulose acetate propionate fibers, or cellulose acetate butyrate fibers.
 16. The method according to claim 1, which further comprises: dispersing the metal oxide particles in the protic liquid to form the mixture; removing at least a portion of the protic liquid from the surface of the fibers after contacting the fibers with the mixture; removing at least a portion of excess or unattached metal oxide particles from the surface of the fibers; and drying the fibers to obtain finished cellulose ester fibers with attached metal oxide particles.
 17. Cellulose ester fibers produced according to the method of claim
 1. 18. Cellulose ester fibers produced according to the method of claim
 16. 19. The cellulose ester fibers according to claim 16, which are substantially free of plasticizer.
 20. A cigarette filter which comprises the cellulose ester fibers according to claim
 16. 